It is well known that bare uncoated fibers are susceptible to abrasion which results in surface scratches. These scratches can result in sudden failure through breakage of the fiber. Optical fibers are susceptible to breakage not only because they are formed from relatively brittle materials, but in addition, the fibers typically have very small diameters and are subjected to an assortment of stresses such as bending stresses, tensile stresses, and the like.
Silica base optical fibers are also susceptible to stress corrosion cracking also known as static fatigue. Microcracks in fiber surfaces present regions which are susceptible to attack by hydroxyl ions or moisture when the fiber is under stress. The combination of stress and chemical attack tends to open these cracks further. Growth of these microcracks weakens the fiber continuously over a period of time until it produces sudden failure. As mentioned above, this problem is termed static fatigue.
One approach to the problem of static fatigue is to surround the fiber with an impervious hermetic coating so that atmospheric moisture cannot reach the fiber surface. Various types of coatings have been investigated. A metallic seal of aluminum has been suggested as a hermetic coating ("Reduction in static fatigue of silica fibers by hermetic jacketing"--Pinnow, Robertson, Wysoski--Appl. Phys. Lett. 34 (1), January 1979), however, metals tend to form polycrystalline solids which can themselves be corroded by moisture or altered by enhanced grain boundary diffusion. Metal coatings also provide electrical paths along the fiber which may be undesirable and have been found to induce optical loss in the fiber through microbending.
Several non-metallic coatings have also been utilized on silica based optical fiber. For example, silicon nitride (U.S. Pat. No. 4,028,080 to DiVita et al) has been investigated as a potential coating, but silicon nitride has been seen to weaken the fiber substantially due to residual stress in the coating. Also, it is difficult to make strong fibers in long lengths with silicon nitride. Pyrolytic carbon has also been suggested in U.S. Pat. No. 4,183,621 to Kao et al and plasma deposited carbon was suggested by Stein et al ("Ion plasma deposition of carbon-indium hermetic coatings for optical fibers"--Proceedings of Conference of Laser and Electro-Optics, Washington, D.C., June 10-12, 1982). In both cases, the stress corrosion coefficient, .eta., of the coating was determined to be in the range of 23-30--essentially no different from that of uncoated silica. This suggests that such coatings are not hermetic. It has recently been reported that silicon carbide may serve as a hermetic coating for silica based optical fiber (U.S. Pat. No. 4,512,629 to Hanson et al). The results of investigation with silicon carbide indicate an n value of 100 or higher can be obtained.
Heavy metal fluoride glass fiber has rapidly developed to be a useful medium for mid-infrared optical transmission, and has great potential for long distance infrared optical communication. The potential of these fibers for ultra-low optical loss promises to increase the repeator spacing in long haul links, especially in undersea cables. The chemical durability of fluoride glasses is low and, therefore, the expected lifetime of fluoride fibers is substantially reduced over that of silica based fibers. Liquid water reacts readily with the fiber surface, dissolves its outer layer and eventually dissolves the entire fluoride fiber. Gaseous, atmospheric water attacks the surface, liberating hydrogen fluoride and introducing hydroxide into the fiber.
Two solutions to the above described problem have been proposed. The first one is to protect the fiber surface by an impervious hermetic coating, to prevent water or moisture from reaching the fiber surface. A common way of protecting fluoride fibers today is by applying a thin layer (i.e., about 5 to 30 microns) of a Teflon FEP coating. This layer improves the fiber handleability but does not protect against moisture migration through the coating.
The second proposed solution involves producing a core glass of mixed fluorides and cladding of chalcogenide glasses. These chalcogenide glasses are known to have better chemical durability than fluoride glasses. (McKenzie et al., Proceeding of Third International Symposium on Halide Glasses, June 24-28, 1985, Rennes, France). The important disadvantage of chalcogenide glass is its high level of toxicity, which is an impediment in medical or surgical applications. Another problem, common for both fluoride and chalcogenide glasses, is the necessity to protect them during the fiber draw by an inert atmosphere to avoid surface reaction with moisture, the creation of defects and initiation of crystallization. (Doremus--Journal of Mat. Sci. 20, 1985); (Sapsford, STL--Proc. for Soc. of Glass Technology, 1986, Scotland).